Wave Trap

ABSTRACT

The patent describes a device intended to both attenuate water waves efficiently, and efficiently convert the attenuated energy to useful energy, while having minimal negative environmental impact. The device consists of a flexible slab that rests upon a rigid but permeable horizontal plane between surface and bottom. The slab is alternatively pushed away from the plane, and alternatively it is pressing hard on the plane. The resulting force may be converted to hydraulic energy using established technology. The critical parameters are the submerged weight per area and elasticity of the slab, equations for which are given in the patent.

BACKGROUND OF THE INVENTION

Waves constitute a problem for navigation, especially at loading and unloading sites why these are often placed in a harbor where they are protected by a breakwater. Floating breakwaters can be used to protect marinas, but there is an upper limit in wave period that those can protect against. Furthermore waves are a potential threat to infrastructure in the open sea, e.g. drilling and production platforms in oil and gas fields, and terminals for LNG and petroleum products.

Waves may also erode beaches so that onshore infrastructure is endangered. Both the probability for erosion, and the economical value of the destruction if erosion does occur, increases if there are buildings on the beach. The intensive building activity on barrier islands and beaches around the world during the past quarter century has therefore led to a dramatic increase in the risk exposure, regardless of whether global warming will lead to a sea-level rise or not.

From environmental reasons it is desirable to be able to attenuate the waves effectively without side effects on the marine or littoral environment. Piers, breakwaters, rip-rap, and groins all have a large environmental impact. The former decrease the water circulation, which leads to the loss of bottom biotopes that are very important or even decisive for the marine environment. The latter destroy the beach environment quite physically, and rip-rap changes the eco-system of the beach permanently.

During strong sea wind some exposed harbors can not be called at due to breaking waves in the harbor entrance. This renders them useless as ports of refuge for vessels at sea, which is unfortunate since it is precisely during strong sea wind conditions that a port of refuge may be most needed. To overcome this problem an outer breakwater may be built on much deeper water, where the waves can not break. Such a breakwater will, however, become quite expensive, and it will significantly limit the water circulation.

For attenuating waves at sea there is a patent that describes a register consisting of a large number of curved wings, which can be mounted under the surface. The intention is that a water flow be created when the waves reach the register. The design is, however, quite complex and thus costly for the scale required. Furthermore it does not lend itself to the generation of renewable energy as a side effect of wave attenuation.

There are existing solutions for energy production that use vertical orbital movements between the surface and the bottom. However, they do not attenuate the waves efficiently.

An existing wave attenuator patent, U.S. Pat. No. 3,197,963 A, describes a device that in one embodiment consists of a rigid and permeable horizontal plane mounted between the surface and the bottom, on the underside of which a partly gas-filled bladder is resting, inside which there are devices to assure that the gas is freely mobile in the propagation direction of the waves. The method of attenuation is that the gas moves inside the bladder. While this can be shown to work empirically, the device is rather expensive to make; and although it is deployed under the surface it has to be placed relatively close to it to be efficient. Finally, even if it does attenuate the waves the energy is lost as turbulence, and no solution is readily available for incorporating that device in a wave energy conversion system.

Cho & Kim (1988, J. Fluid Mech., 367:139-161) describe a submerged flexible membrane that is kept stretched out horizontally by applying a suitable force to its edge. Although it may attenuate the waves and split the wave period in half it does not lend itself to be used in a wave energy converter for power generation. Furthermore a practical and economical implementation of it is difficult due to the nature of the materials and forces required.

BRIEF SUMMARY OF THE INVENTION

The challenge is to efficiently attenuate the waves without diminishing the water circulation, without intruding on the beach environment, without hindering navigation and shipping, and in such a way that the wave energy can be converted to useful energy. The goal of the present invention is to both attenuate the waves efficiently and to efficiently convert the attenuated energy to useful energy—and to accomplish this feat with minimal negative environmental impact.

The invention consists of a horizontal device mounted between the surface and the bottom. It consists of a flexible slab, having a certain combination of submerged weight per area and modulus of elasticity, which rests on a permeable, rigid, and plane net, grid, or equivalent (herein referred to as the plane). The slab can be pushed away from the plane during part of the wave phase, but is stopped in its movement by the plane during the opposite part of the cycle. The pressure that the waves thus exert on the device is converted to forces within its mechanical structure, which can be further converted to commercial energy using known technology. At the same time it has a simple construction and can be built using relatively low-cost materials. Like the flexible membrane it can split the wave period in half, which can make it possible to use a floating breakwater after it for attenuating the remaining smaller waves.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical section through the active device consisting of a mesh (1) and a sheet (2), located between water surface and bottom. It is drawn perpendicular to the wave motion, which is from left to right as illustrated by the wave orbitals. The device is similar along its entire width (the cross-paper direction).

FIG. 2 illustrates the definitions of the symbols used in the equations, along with an alternative implementation of the invention where the sheet is under the mesh instead of above it.

FIG. 3 is a vertical cross-section through the legs, of an alternative implementation using supporting legs (3) and pistons (5) attached by hinges (4) to anchor the device to the bottom.

DETAILED DESCRIPTION OF THE INVENTION

The wave energy conversion is accomplished by breaking the orbital motion in the water column during a part of the wave phase, whereby a pressure difference results. Refer to FIG. 1. The device consists of a rigid but permeable plane (1), located approximately horizontally at a depth between the water surface and bottom, upon which a flexible slab (2) rests kept in place by gravitation, and optionally fastening devices, for example ropes, that prevent it from sliding horizontally with the waves. If the slab is heavier than water it is placed above the plane as in FIG. 1, but if it is lighter it is placed below as in FIG. 2.

When the orbital motion presses the slab away from the plane, as at phase angle 270° in FIG. 1, the slab yields. Since the slab is resting on the underlying water the pressure of the latter increases somewhat. At phase angle 90° the opposite conditions exist; the plane stops the slab so that the orbital motion meets rigid resistance. A relative low pressure is thus created under the device. Even when the slab is lighter than water, as in FIG. 2, the pressure is higher under the slab when it is at the highest. This follows logically by the fact that the pressed-down slab is exercising a negative pressure under itself. The motion of the slab comes to an abrupt halt when it hits the plane, which creates the empirically determined good wave energy conversion rate of the device.

FIG. 2 defines the terms used in the following design recommendations, which are based on empirical data. The device will work also if one or several of these specifications are exceeded, but if all are followed it is expected to convert at least 80% of the energy from the design waves.

The water depth (h) should be 15% to 50% of the wavelength (λ). The mounting depth of the device (d) should be 33% to 67% of the water depth (h). Efficient conversion can result when the pressure variations at the device are between 30% and 70% of those at the surface.

The length (L) of the device must significantly exceed half the wavelength (λ), and is recommended to exceed 70%. It should furthermore be avoided to have a phase difference from one side to the other of the device that is close to π radians; recommended values are from −150° till +150°. The width of the device, defined as its extent along the wave crests, is selected according to the desired protection.

The openness of the plane ought not be less than 50% and should preferably be above 90%. The plane may be constructed from just about any material ranging from expanded metal to concrete beams, depending on scale.

The following equation, which is based on the quotient between the weight of the slab and the hydrostatic pressure, can be used to determine the maximal permissible thickness of the slab:

$t < {\frac{H\; \rho_{w}{\cosh\left( \frac{2{\pi \left( {h - d} \right)}}{\lambda} \right)}}{2\left( {\rho_{s} - \rho_{w}} \right){\cosh\left( \frac{2\pi \; h}{\lambda} \right)}}}$

where ρ_(s) and ρ_(w) is the density of the slab and the water, respectively

The recommended thickness is 40% to 80% of the maximal. The slab must also be so flexible that its stiffness does not prevent the waves from pressing it away from the plane. The following equation may be used to calculate its maximal modulus of elasticity:

E<0.015625λ⁴ρ_(w) gt ⁻³ where g is the gravity

As examples of functioning slab materials can be mentioned closed cell foam, but also inflatable air bags will work if they are constructed so that the air is distributed so evenly over the area that the character of slab is retained, which among other things requires that the air can not move significantly over a single wave period. Sheet lead does also work due to its low modulus of elasticity in relation to density. For longer waves many materials with a density that is significantly different from that of water should work, and for really long waves even concrete may be considered.

The plane must be anchored to the bottom. This can be done using piles (3). If the lifting force of the slab significantly exceeds the weight of the plane the device can alternatively be hung up using chains, wire, or rope from anchors on the bottom.

The device herein described tends to double the wave frequency and thus to cut their period in half, at the same time as the wave height is drastically reduced. The new waves are created in the trough of the original ones. This may happen at the inner edge of the device at the arrival of a compression wave underneath the device, or along the sides when pressure is leaking out from a compression wave propagating underneath the device.

Further damping might be achievable by allowing the pressure difference across the slab to generate a limited flow of water through the slab. This limits the generation of waves along the sides and at the inner edge. Such a flow may be created by making the slab in segments that have full length but limited width, meeting edge in edge or with a narrow gap in between. A leakage is thus created so that pressure is released gradually over the entire device.

In order to get optimal effect regardless of the sea state it may be advantageous to make some design parameters adjustable. This can for instance be achieved by raising and lowering the device, or by changing the amount of air in an inflatable slab. Also the effective length, width, or thickness of the slab may be changed if it is subdivided in cells, since the damping caused by empty cells is negligible.

The plane (1) can be made as a single rigid constructional element as in FIG. 1 and FIG. 2, or as several elements connected by hinges (4) as in FIG. 3. If the plane is attached with hinges the pressure that the slab is rhythmically developing against the plane can be used to carry out work, for instance in a pump (5) attached to the bottom. An equivalent solution may be arranged if the device is lighter than water and hung up in a wire or similar from the bottom. 

1. A device for capturing energy from sea waves, consisting of a flexible slab, the density of which is distinct from that of water, and which rests on or under a permeable, rigid, and roughly horizontal plane, located at such a depth between the surface and the bottom that the waves are able to press the slab away from the plane during a part of the wave phase, which requires that the slab has a submerged weight per area that is nonzero but the absolute value of which is less than the absolute value of the maximal hydrostatic pressure on it caused by the waves; furthermore the device is characterized by the slab and plane having an extent in the direction of propagation of the waves of at least half the wavelength of the incoming waves.
 2. The same device as in 1 where the plane consists of hinged segments, and where the plane has been anchored in the bottom in such a way that one or several attachment legs are exposed to vertical forces that can be converted to useful work in a device that can be driven by linear motion.
 3. The same device as in 1 or 2 where the elevation of the device is adjustable so as to allow for the wave attenuation efficiency to be optimized. 